The easy way?...I'd use something like this in a 1/2" drill...http://cgi.ebay.com/Taper-Pin-Hole-R...item4ced84853d
With more elbow grease, you can accomplish the same result with a large rat tail file...http://cgi.ebay.com/HAND-FILE-JOHNSO...item3a42986b32
OR a grinding burr in a dremel.